Binary asteroids - two rocky objects orbiting one another - appear to be common in Earth-crossing orbits, say astronomers.

Using the world's two most powerful astronomical radar telescopes, researchers say these double asteroid systems are formed when single asteroids are torn apart during encounters with the Earth.

Writing in the journal Science, the scientists estimate that about 16% of so-called near-Earth asteroids (NEAs) larger than 200 metres (656 feet) in diameter are likely to be double systems.

To date, five such binary systems have been identified by radar.

Jean-Luc Margot of the California Institute of Technology, US, says theoretical studies suggest that binary asteroids appear to be formed extremely close to Earth - within a distance equal to a few times our planet's radius.

Nudged towards Earth

NEAs form in the asteroid belt, between the orbits of Mars and Jupiter, and are nudged by the gravity of nearby planets, largely Jupiter, into orbits that take them to the Earth's neighbourhood.

In 2000, Margot and colleagues, using measurements from the Goldstone radar telescope, discovered that a small, roughly 800-metre-diameter (half-mile) asteroid, 2000 DP107, was a binary system.

Observations made last October with the much more sensitive Arecibo radar telescope established the physical characteristics of DP107's two asteroids as well as their orbit about each other.

The smaller object is about 300 metres (1,000 feet) in diameter and is orbiting the larger asteroid every 42 hours at a distance of 2.6 kilometres (1.6 miles). The two asteroids appear to be locked in synchronous rotation, with the smaller rocky body always with the same face turned towards to the larger body.

Since DP107 was discovered, four more binary NEAs have been found, all in Earth-crossing orbits and each with a main asteroid significantly larger than the smaller body.

Double impact

Astronomers have long suspected that there could be binary NEAs. Of about 28 known terrestrial impact craters with diameters greater than 20 kilometres, at least three are double craters formed by impacts of objects about the same size as the newly discovered binaries.

Also, astronomers have noted the changes in the brightness of some NEAs, indicating a double system is causing an eclipse or occultation of one by the other.

NEAs, basically piles of rubble held together by gravity, are on trajectories that bring them within a few thousand kilometres of the planets, where tidal forces can increase the spin rate of the asteroids, causing them to fly apart. The ejected rubble then reforms in orbit around the larger object.

There is an important reason for studying binary asteroids, says Nasa's Steve Ostro: their potential for colliding with Earth.

He says that knowing the density of potentially hazardous asteroids "is an extremely important input to any mitigation plans".

"Getting NEA densities from radar is dirt cheap compared with getting a density with a spacecraft."

Jean-Luc Margot says: "Radar gives us very precise measurements of the size of the objects and their shape.

"From this we can obtain the mass of each of the objects allowing, for the first time, measurements of NEA densities, a very important indicator of their composition and internal structure."